1. Field of the Invention
This invention relates generally to power converters and, more particularly, to double-switched flyback converters.
2. Related Art
Modern electronic systems require power regulation to condition the output of the primary power source. Power converters provide a regulated DC voltage from a primary DC source. Conventional DC/DC converters are generally of the push-pull or switching regulator type.
A flyback converter uses a switching regulator to cyclically store and transfer energy from a primary DC source through a power transformer to an output circuit. The primary winding of the power transformer is connected to the primary DC source; the secondary winding is connected to the output circuit. The switching regulator alternately opens and closes a switch connecting the primary DC source to the primary winding of the transformer. This switching action defines a power cycle which inductively couples the primary DC source to the output circuit.
The power cycle of the converter comprises two parts: a primary conduction cycle and a flyback cycle. During the primary conduction cycle, the switch is closed, transferring energy from the primary DC source to the primary winding of the power transformer. At a preset power level in the primary winding, the switch is opened, starting the flyback cycle. During this cycle, energy is coupled through the secondary winding into a rectified output circuit.
The amount of energy stored per cycle is controlled by regulating the switching action. Generally speaking, a pulse width modulating (PWM) circuit regulates the switching action by varying the pulse width or "on" time of the switches. Negative feedback of the output voltage is frequently used in conjunction with the PWM circuit to provide output voltage regulation.
A double-switched flyback converter uses two switches in series and on opposite ends of the primary winding of the transformer. The switches, typically, are high voltage transistors driven by a gate drive transformer connected directly to the PWM circuit. The PWM circuit, acting through the gate drive transformer, biases the gates of the transistor switches to the "on" state, storing energy in the primary winding during the primary conduction cycle. The gates of the transistor switches are subsequently biased to the "off" state during the flyback cycle.
In known converters, transients in the primary DC source or output load may impose undesirable stresses in the converter circuit. For instance, a surge in the input voltage may transfer excess energy to the primary winding during the primary conduction cycle; during the subsequent flyback cycle, the PWM circuit may command the transistor switches "on" before the energy transfer to the secondary winding is complete. This condition imposes high stress currents on both the transistor switches and the power transformer, leading to possible component failure. This condition occurs because the PWM circuit is blind to the exact status of the energy transfer at the power transformer. Therefore, it is desirable to monitor the status of the energy transfer at the transformer windings to provide for more efficient switching action.